Nuclear magnetic resonance (NMR) technologies can be useful in a wide variety of applications. For example, in the field of oilfield services, NMR logging tools can provide information regarding fluids in a formation as well as porosity of the formation. Such information can be combined with data collected using other technologies to better inform engineers as they engage in various pursuits including, for example, formation evaluation, completion engineering, geological characterization, reservoir production, etc.
NMR logging tools can be introduced into a wellbore in a variety of ways. For example, an NMR logging tool can be included in a bottom hole assembly and take measurements during a drilling operation. NMR logging tools can also be lowered into a wellbore using other technologies, such as wireline technologies.
General background of nuclear magnetic resonance (NMR) well logging is set forth, for example, in U.S. Pat. No. 5,023,551. Briefly, in conventional NMR operation, the spins of nuclei align themselves along an externally applied static magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. a radio frequency (RF) pulse), which tips the spins away from the static field direction. After tipping, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by ω0=γ×B0, where B0 is the strength of the static field and γ is the gyromagnetic ratio. Second, the spins return to the equilibrium direction according to a decay time T1, which is called the longitudinal relaxation time constant or spin lattice relaxation time constant. For hydrogen nuclei, γ/2π=4258 Hz/Gauss, so, for example, for a static field of 235 Gauss, the frequency of precession would be 1 MHz. Also associated with the spin of molecular nuclei is a second relaxation time constant, T2, called the transverse relaxation time constant or spin-spin relaxation time constant. At the end of a ninety degree tipping pulse, all the spins are pointed in a common direction perpendicular to the static field, and they all precess at the Larmor frequency. The net processing magnetization decays with a time constant T2 because the individual spins rotate at different rates and lose their common phase. At the molecular level, dephasing is caused by random motions of the spins. The magnetic fields of neighboring spins and nearby paramagnetic centers appear as randomly fluctuating magnetic fields to the spins in random motion. In an inhomogeneous field, spins at different locations precess at different rates. Therefore, in addition to the molecular spin-spin relaxation of fluids, spatial inhomogeneities of the applied field also cause dephasing. Spatial inhomogeneities in the field can be due to microscopic inhomogeneities in the magnetic susceptibility of rock grains or due to the macroscopic features of the magnet.
A widely used technique for acquiring NMR data, both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, a ninety degree pulse causes the spins to start processing. Then a one-hundred-eighty degree pulse is applied to cause the spins which are dephasing in the transverse plane to refocus. By repeatedly refocusing the spins using one-hundred-eighty-degree pulses, a series of “spin echoes” appear, and the train of echoes is measured and processed. The transverse relaxation time constant, T2, or the distribution of multiple T2s, can be reliably obtained using this technique. In well logging, the CPMG sequence is traditionally executed using a set of equipment located “downhole” in a wellbore (in situ). While performing the CPMG sequence in situ allows for relatively rapid data gathering, limitations of the equipment and the environment can make it difficult to obtain accurate downhole data. For example, due to the limits on equipment power, design constraints and downhole conditions, the signal to noise ratio (SNR) for an in situ CPMG sequence remains low. In addition, while the CPMG sequence may be useful in measuring a T2 distribution which correlates with the properties of a reservoir fluid, the CPMG sequence is not very well suited for studying solid samples with strong dipolar interactions because the pi (π) pulse rotations that make up the CPMG sequence do not refocus the homonuclear dipole-dipole interaction between nearby hydrogen atoms.
More particularly, solids are characterized by short transverse coherence times due to the presence of molecular interactions (which are generally averaged out in liquids due to Brownian motions). These anisotropic molecular interactions, such as dipole-dipole interactions, result in broadening of the NMR spectral lines (peaks), or shortening of the transverse relaxation times. For example, the Hamiltonian for the magnetic dipole-dipole interaction between two nuclear spins I1 and I2 separated by an inter-nuclear distance r is given by
                              D          =                                                                      μ                  0                                ⁢                                  γ                  1                                ⁢                                  γ                  1                                ⁢                                  h                  2                                                            16                ⁢                                  π                  3                                                      ⁡                          [                                                                                                                  (                                                                                                            I                              1                                                        _                                                    ⁢                                                                                                          ⁢                                                                                    I                              1                                                        _                                                                          )                                                                    r                        3                                                                                                                                                3                        ⁢                                                  (                                                                                                                    I                                1                                                            _                                                        ⁢                                                                                                                  ⁢                            r                                                    )                                                ⁢                                                  (                                                                                                                    I                                2                                                            _                                                        ⁢                                                                                                                  ⁢                            r                                                    )                                                                                            r                        5                                                                                                        ]                                      ,                            (        1        )            where μ0 is the vacuum permeability, and h is the Plank's constant. When multiple hydrogen atoms are present, the total dipolar Hamiltonian is a sum of equation (1) for all pairs. In the laboratory, the line broadening due to the dipolar coupling in solids can be partly overcome by using Magic Angle Spinning (MAS) technique. See, e.g., E. R. Andrew, “Magic Angle Spinning”, Solid Slate NMR Studies of Biopolymers, John Wiley & Sons. pp. 83-97 (2010). This technique involves averaging out of the interactions by rapidly spinning the samples along an axis at a particular angle (˜54°) relative to B0, resulting in a drastic reduction in the line widths. MAS has been extensively used for the study of oilfield solids and viscous fluids such as kerogen, bitumen and heavy oils in laboratories. However, this technique cannot be implemented downhole in well-logging.
In NMR well-logging tools, depending upon the tool, the shortest echo time available may be between 200 μs and 1050 μs. Since the relaxation time of shale samples containing kerogen can be as short as 100 μs due to strong dipolar interaction, such a short relaxation component cannot be accurately measured by the current logging tools.